Flat tube for heat exchanger

ABSTRACT

A tube ( 10 ) for a motor vehicle heat exchanger exhibits a cross section comprising two opposite longitudinal walls ( 12 ), each provided with at least two concave portions ( 14 ).

The present invention relates to the technical field of motor vehicleheat exchangers and relates more specifically to a tube for a heatexchanger of this kind.

The exchanger tubes concerned are so-called “flat” tubes intended topass through the openings in a plurality of fins of the heat exchanger.The tubes are shaped in the heat exchanger by expansion, so that theyare pressed against the edges of the openings in the fins.

A tube of the kind referred to is already known from document FR 2 722563, said tube exhibiting an oblong cross section which is substantiallyequivalent to that of an opening in a fin.

A tube of this kind comprises two longitudinal walls facing one anotherwhich are curved and exhibit a concave portion (in other words, aportion having a convexity turned towards the inside of the tube), eventhough the tube has a restricted outer width in a region in which thelongitudinal walls are closest one to the other. This kind ofparticularly flat tube is therefore adapted to be located in asmall-size heat exchanger.

However, once a tube of this kind has been shaped in an exchanger,cohesion problems have been noticed between the longitudinal walls ofthe tube and the openings in the fins. Tubes of this kind are subject toan elastic spring back of their walls after shaping in the openings ofthe fins. This elastic springback of the walls of the tube creates playbetween the walls and the openings in the fins which results in areduction in heat conductance between the tube and the fins. These kindsof deficiencies therefore have a negative impact on the overall thermalefficiency of the exchanger.

The particular aim of the present invention is to overcome theaforementioned problems by proposing a heat exchanger tube, the thermalefficiency of which is improved.

To this end, the object of the invention is a tube for a motor vehicleheat exchanger exhibiting a cross section comprising two oppositelongitudinal walls, each wall being provided with at least two concaveportions.

Hence, a particularly flat tube is proposed which exhibits dimensionsadapted to be fitted in a small-size heat exchanger, while eliminatingthe cohesion problems between the walls of the tube and the opening inthe fin. Surprisingly, the inventors have noticed that this particulartube shape allows an elastic springback of the walls of the tube to beavoided following its shaping by expansion. In fact, following theshaping of a tube, the sides of the tube remain pressed against theedges of the opening and prevent the problem of play emerging betweenthe openings in the fins and the walls of the tube. The conductance ofheat between the fins and the tube is therefore optimized, which therebyimproves the thermal efficiency of the exchanger.

The tube as defined above may, moreover, comprise one or more of thefollowing characteristics, either alone or in combination.

According to the invention, the two concave portions are separated by aconvex portion. A shape of this kind should be understood in thebroadest sense, in other words without the convex portion having to havea particular shape.

The convex portion is of such a size that the internal perimeter of thesection has a value that is at least 2% greater than the section of atube in which the walls are straight. Hence, the convex portion allowsthe internal perimeter of the tube, also referred to as the wettedperimeter, to be increased by a value of 2% in respect of a tube thatdoes not exhibit any convexity, the result of this being an increase inthe tube's heat exchange performance.

For example, the walls of the tube are connected to one another bysections which are substantially circular or even oval in shape.

It can be envisaged that, according to the cross section of the tube, alength L_(Tube) denotes the greatest distance between the sections witha substantially circular shape on the outside of the tube, a widthI_(Tube) denotes the smallest distance between the two walls of the tubeon the outside of the tube and a flatness ratio T_(f) of the tube isdefined by the relationship:

$T_{f} = \frac{l_{Tube}}{L_{Tube}}$

where the flatness coefficient Tf is strictly lower than 0.20 to ±0.01.

Another object of the invention is a fin comprising at least onesubstantially oblong opening which comprises an edge provided with twoopposite longitudinal walls, each one provided with at least two convexportions. The specific shape of a fin of this kind is particularlyappropriate for avoiding an elastic springback of the walls of the tubefollowing shaping in an opening in the fin.

-   -   It can be envisaged that the two convex portions of the fin are        separated by a concave portion. A shape of this kind contributes        to good cohesion between the walls of the tube and the opening        in the fin.    -   Another object of the invention is a heat exchanger provided        with at least one fin comprising at least one tube such as that        described above. An exchanger provided with fins and tubes such        as those defined above thereby exhibits an improved heat        exchange performance.    -   The final object of the invention is an assembly comprising at        least one tube such as the one above and at least one fin in        which the tube is configured to be introduced into the inside of        an opening in the fin with play existing between the tube and        the opening.

The invention will be easier to understand on reading the attachedfigures, which are provided by way of example and are in no way limitingin nature, in which:

FIG. 1 is a sectional view of a tube according to the invention;

FIG. 2 is an elevation of a portion of a fin intended to receive a tubein FIG. 1.

With reference to FIG. 1, a tube 10 according to the invention isdepicted prior to its shaping by deformation. Said tube exhibits asubstantially oblong cross section comprising longitudinal walls 12facing one another, each wall being provided with at least two concaveportions 14. In other words, viewed from the outside of the tube, thetube 10 comprises two hollow forms 14 which are substantially curved.

The two longitudinal walls 12 are connected to one another by two endsections 16 which are circular in shape. In the example, a convexportion 18 is arranged between the two concave portions 14 of eachlongitudinal wall 12. Hence, viewed from the outside, each wall 12 ofthe tube exhibits a raised section arranged between the two hollowshapes 14.

The tube 10 is produced from a metal alloy adapted to be easily deformedand has a wall with a substantially constant thickness e.

Along the cross section, the tube has a length L_(Tube) denoting thegreatest distance separating the two end sections 16 which aresubstantially circular in shape on the outside of the tube and a widthI_(Tube) denoting the shortest distance between the two walls 12 of thetube on the outside of the tube. A flatness ratio T_(f) of the tube canbe defined by the relationship:

$T_{f} = \frac{l_{Tube}}{L_{Tube}}$

where the flatness coefficient T_(f) is strictly smaller than 0.20 to±0.01.

With reference to FIG. 2, a fin 20 which is generally rectangular inshape comprises a plurality of holes 22 disposed in a row along axis N.The holes 22 in a fin 20 are separated in twos by louvered rows 24 wellknown to the person skilled in the art, the function of which is toincrease the heat exchange of the fins 20.

Each opening 22 exhibits a cross section which is substantially oblongin shape, comprising two longitudinal edges 26 facing one another, eachedge 26 being provided with at least two convex portions 28. The twoconvex portions 28 of an edge 26 of the opening 22 are separated by aconcave portion 30. Viewed from the inside of the opening, eachlongitudinal edge exhibits one hollow section and two raised sections.

Each opening 22 in a fin 20 therefore has a shape that is substantiallyidentical to that of a tube 10, but its dimensions are smaller, in orderto allow the tube to be threaded inside the opening 22 prior to shaping.Hence, each opening is configured to receive a tube as previouslydescribed, in such a manner that an assembly formed by an openingprovided with a tube exhibits play J between the tube and the receivingopening that matches it.

There follows a description of a method of shaping a tube 10 accordingto the invention in a plurality of fins 20 of a heat exchanger. In afirst stage, the tube 10 is positioned in an opening 22 of at least onefin 20, in such a manner that the tube 10 is positioned to create play Jbetween the tube and the opening, said play J not having a consistentvalue over the entire periphery of the tube. The tube is then made topress against the inside of the opening 22 in the fin due to theexpansion of its walls.

The tube and the fin that are presented are not limited to theembodiment described above and other embodiments will appear clearly tothe person skilled in the art.

1. A tube for a motor vehicle heat exchanger exhibiting a cross sectioncomprising two opposite longitudinal walls, wherein each wall isprovided with at least two concave portions.
 2. The tube as claimed inclaim 1, wherein two concave portions are separated by a convex portion.3. The tube as claimed in claim 2, wherein the convex portion is of sucha size that the internal perimeter of the section has a value that is atleast 2% greater than the section of a tube in which the walls arestraight.
 4. The tube as claimed in claim 1, wherein the walls of thetube are connected to one another by end sections which aresubstantially circular in shape.
 5. The tube as claimed in claim 4,exhibiting, according to the cross section of the tube, a lengthL_(Tube) denoting the greatest distance between the sections with asubstantially circular shape on the outside of the tube, a widthI_(Tube) denoting the smallest distance between the two walls of thetube on the outside of the tube and a flatness ratio T_(f) of the tubedefined by the relationship: $T_{f} = \frac{l_{Tube}}{L_{Tube}}$ wherethe flatness coefficient Tf is strictly lower than 0.20 to ±0.01.
 6. Afin comprising at least one substantially oblong opening, wherein theopening comprises an edge provided with two opposite longitudinal walls,each provided with at least two convex portions.
 7. The fin as claimedin claim 6, in which two convex portions are separated by a concaveportion.
 8. A heat exchanger provided with at least one fin as claimedin claim 7 and comprising at least one tube for a motor vehicle heatexchanger exhibiting a cross section comprising two oppositelongitudinal walls, wherein each wall is provided with at least twoconcave portions.
 9. An assembly of at least one tube for a motorvehicle heat exchanger exhibiting a cross section comprising twoopposite longitudinal walls, wherein each wall is provided with at leasttwo concave portions and at least one fin as claimed in claim 6, whereinthe tube is configured to be introduced into the inside of an opening inthe fin with play existing between the tube and the opening.